1. Field of the Invention
This invention relates to a prism for separating the illuminated light from a light source depending on the wavelength or polarized light components, a projection device for modulating the illuminated light radiated from the light source using a reflection type optical modulating element and for projecting the light to an enlarged scale using a lens and an optical component used in this prism and in the projection device.
2. Description of Related Art
For enabling display on a large sized screen, there has so far been known a projection device in which illuminated light is radiated from a lamp to a liquid crystal panel, on which a pattern consistent with input picture signals is displayed, and in which the illuminated light is modulated and reflected by the liquid crystal panel so as to be projected to an enlarged scale using a projector lens.
In this projection device, a polarized beam splitter (PBS) for separating the forward and return optical paths is provided in the optical path so that the optical path for the illuminated light towards the liquid crystal panel and that for the reflected light modulated by the liquid crystal panel will not be the same optical path. In this PBS 200, a pair of corner prisms 202, as substrates, is bonded together to hold a dielectric multi-layer film 201 in-between as shown in FIG. 1. Moreover, in this PBS 200, the dielectric multi-layer film 201 is formed to have different reflectance and transmittance values, depending on the wavelength or the direction of light polarization, and operates as a beam splitter for separating a light beam depending on the wavelength or the direction of light polarization.
The PBS 200 can be classified into a McNeill type in which the dielectric multi-layer film 201 separates the light beam depending on the polarized light component, and a dichroic type, in which the light beam is separated depending on the wavelength.
In the McNeill or dichroic type PBS, exploiting the interference of multi-layer dielectric films, the performance is determined by the refractive index of the substrate and the combination of the layered dielectric materials, so that the performance beyond a certain limit cannot be achieved. In such PBS, it is extremely difficult to maintain separation characteristics between the P-polarized light and the S-polarized light over a wide angle of incidence. Thus, if this PBS is built into an optical system with a large angular distribution, that is with a small F-value, light separation characteristics as well as the light exploitation efficiently is lowered.
As a solution, a flat plate shaped diffraction grid PBS 210, shown in FIG. 2, may be used. In this diffraction grid PBS 210, a diffraction grid 212 of e.g., aluminum is provided on a glass substrate 211. By this diffraction grid 212, light is separated depending on polarized components. However, if the grid is used as a beam splitter, the diffraction grid PBS 210 needs to be arranged obliquely relative to the main light beam, so that, if the diffraction grid PBS is inserted into the optical path, astigmatic aberration is produced.
In the case of the McNeill or dichroic prism, distortion is produced in the prism due to temperature rise in the prism or by a holding mechanism, so that diffraction distribution in the substrate becomes non-uniform with the result that phase difference is produced in the light transmitted through the substrate and hence the extinction ratio is partially lowered. With the projection device, employing this prism, so-called black spots are produced in the projected image such that clear images cannot be projected. Thus, as a prism substrate, such a material having a low modulus of opto-elasticity and less susceptible to distortion in the prism needs to be selected. However, a material having a low modulus of opto-elasticity is costly, such that the prism employing the low modulus of opto-elasticity or the projection device employing such prism is expensive. On the other hand, it is extremely difficult to produce a prism of high performance from an inexpensive vitreous material having a high modulus of opto-elasticity.